1. Field of the Invention
The present invention relates to a traveling-wave amplifier (TWA) formed by using a semiconductor process.
2. Description of the Related Art
With the recent rapid increase in traffic of data communication, there has been a continuous demand for further increasing the capacity of communication networks and the transmission rate of communication signals. Accordingly, optical transmission systems having an increased transmission rate of communication signals from 10 Gbps to 40 Gbps and further to 100 Gbps are being developed, for example.
The TWAs are excellent in terms of high-speed response. Therefore, in an optical transmitter (optical transmission unit) of an optical transmission system, for example, a TWA is used to drive an optical modulation device, a laser diode, or the like that converts an electrical signal into an optical signal. In general, in a case of attaining a high gain by combining a plurality of amplifiers (amplifier cells), an operating frequency (bandwidth) of a wider range can be set for a TWA than for an amplifier including the same number of amplifier cells as the TWA, the amplifier cells being arranged in a cascade connection.
FIG. 7 illustrates a comparative example of an equivalent circuit diagram. As illustrated in FIG. 7, a TWA 101 includes input lines 111 and 112 that are connected to a plurality of amplifier cells 110, and output lines 121 and 122 that are connected to the plurality of amplifier cells 110. The plurality of amplifier cells 110 has the same characteristics. The input lines 111 and 112 and the output lines 121 and 122 are transmission lines having predetermined characteristic impedance. For example, rectangular symbols 111a, 112a, 121a, and 122a illustrated in FIG. 7 each represent an equivalent circuit of a corresponding transmission line per predetermined length. The equivalent circuit of the input line 111 is configured by using symbols 111a that are connected in series, for example.
FIG. 8 illustrates an example of an equivalent circuit where a single signal is input to the TWA 101 illustrated in FIG. 7, and a single signal is output from the TWA 101 in response to the single signal that is input. A circuit operation in which each amplifier cell 110 receives an input signal, amplifies the input signal, and outputs the resulting signal as a part of an output signal is called a single-ended operation. A circuit that performs a single-ended operation is called a single-ended circuit. In FIG. 8, to a connecting node between adjacent symbols 111a of the input line 111, an input terminal 110a of the amplifier cell 110 is connected. To a connecting node between adjacent symbols 121a of the output line 121, an output terminal 110b of the amplifier cell 110 is connected. Here, it is assumed that the wiring resistance Rin of the input line 111 and the wiring resistance Rout of the output line 121 are assumed to be 0, and the gate capacitance Cgs and the drain capacitance Cds of a transistor Tr in the amplifier cell 110 are taken into consideration. In this case, the characteristic impedance Zin of the input line 111 and the characteristic impedance Zout of the output line 121 are respectively calculated by using Eq. 1 and Eq. 2 below, where the capacitance of the input line 111 is denoted by Cin, the inductance of the input line 111 is denoted by Lin, the capacitance of the output line 121 is denoted by Cout, and the inductance of the output line 121 is denoted by Lout.Zin=√{square root over (Lin/(Cin+Cgs))}  (1)Zout=√{square root over (Lout/(Cout+Cds))}  (2)
On the other hand, the input line 111 can be regarded as LC filters arranged in a cascade connection of n stages, and the cut-off frequency f of the input line 111 is calculated by using Eq. 3 below. In Eq. 3, as the number of stages n of the LC filters increases, the cut-off frequency f becomes lower. As the length of the input line 111 increases, the inductance Lin becomes larger, and therefore, the cut-off frequency f becomes lower. The DC resistance of the input line 111 is equal to n×Rin, and therefore, the DC resistance becomes larger as the number of stages n increases. Note that the number of stages n is equal to the number of the amplifier cells 110 included in the TWA 101.f=1/(√{square root over (Lin(Cin+Cgs))}×√{square root over (n)})  (3)
The input line 111 and the output line 121 of the TWA 101 are formed as coplanar lines. A coplanar line having desired characteristic impedance is formed by adjusting the inductance, the capacitance, and the like of the coplanar line. FIG. 9 illustrates a cross-section taken along line IX-IX of FIG. 7. In FIG. 9, the input line 111 is formed as wiring (interior wiring) on a semiconductor substrate 31, and the output line 121 is formed as plated wiring on an insulating layer 32 that is provided on the input line 111.
The above-described TWA 101 may he required to output a driving signal having large amplitude. For example, a Mach-Zehnder modulator (MZM), which is one type of optical modulation device, requires a driving signal having amplitude ranging from 2.5 V to 8 V. In a case where the TWA 101 is used to drive such an MZM, the number n of the amplifier cells 110 of the TWA 101 is increased to thereby increase the gain (total gain). In this case, however, the input line 111 and the output line 121 of the TWA 101 have a longer length of 2 mm to 6 mm, for example. Accordingly, as the number of stages n of the amplifier cells 110 increases, the input line 111 becomes longer, and the cut-off frequency f of the input line 111 decreases, resulting in degradation of high-speed response of the TWA 101. In order to restrain degradation of high-speed response, it is better to decrease the line width (breadth) of the input line 111 and to decrease the parasitic capacitance Cin of the input line 111. In this case, however, the cross-section area of the face of the input line 111 perpendicular to the traveling direction of signals decreases, and the DC resistance of the input line 111 increases. It is difficult to make the input line 111 thicker in order to make up for such shortcomings because the flatness of the insulating layer 32 provided on the input line 111 is a trade-off and because the aspect ratio in production is limited.
In a case of thickening metal wiring that constitutes the output line 121 in order to increase the current-carrying capacity of the output line 121, the aspect ratio is limited in production so that the line width of the metal wiring that constitutes the output line 121 unsuitably increases. As a result, an area needed to form plated wiring of the output line 121 increases. On the other hand, downsizing of an apparatus (optical transmission system, for example) having the TWA 101 and high-density integration of serviced channels are required. Furthermore, the TWA 101 used in the apparatus is required to be downsized and to attain high performance. Therefore, it is desired that, in the TWA 101, the current-carrying capacity of the output line 121 is increased and an area occupied by the output line 121 on the semiconductor substrate 31 is reduced.